Prolonged chilling under moderate light: effect on photosynthetic activity measured with the photoacoustic method

Abstract Exposure of tomato plants to a mild chilling temperature and relatively low ambient photon flux density for an extended period (10°C and 400 μmol photons m^?2 s^?1 d and 5°C night for 6 d) resulted in a significant decrease in the variable chlorophyll fluorescence, the quantum yield of oxygen evolution and the amount of total absorbed energy stored in photochemical intermediates, but not in the chlorophyll concentration or in the activity of ribulose biphosphate carboxylase. These results indicate that photochemical processes involving PSII were affected, and might reflect photoinhibitory effects on the photosynthetic apparatus. Chilling treatment had relatively small influence on the maximal extent of the Emerson effect. This observation, together with the sharp decrease found in the quantum yield of oxygen evolution, could be reconciled with the above results only if some dependency between the two photosystems was assumed. On the basis of this interpretation, it was concluded that the strong Emerson effect after chilling still reflects the typical imbalance between PSI and PSII centres, even though populations of such unaffected pairs are smaller than in the untreated plants. The relatively new photoacoustic technique employed in this study is shown to be useful both as a diagnostic tool and as a means of investigating changes in photochemical activity in the study of environmental stress effects on photosynthesis. The results support the view that photoinhibition can play an important role in limiting photosynthetic activity, and therefore productivity, in chilling-sensitive plants such as the tomato under the natural conditions that prevail during the winter in mediterranean climates.